The invention relates generally to integrated circuits and, in particular, to structures for field-effect transistors and methods of forming such structures.
Complementary metal-oxide-semiconductor (CMOS) technology finds use in, among other applications, radiofrequency (RF) circuits and high speed digital circuits. Generally, CMOS technology relies on complementary and symmetrical pairs of p-type and n-type field-effect transistors (nFETs and pFETS) to implement logic functions. Field-effect transistors include an active semiconductor region, a source and a drain defined in the active semiconductor region, and a gate electrode. When a control voltage exceeding a characteristic threshold voltage is applied to the gate electrode, an inversion or depletion layer is formed in a channel defined in the active semiconductor region between the source and drain by the resultant electric field and carrier flow occurs between the source and drain to produce a device output current.
Semiconductor-on-insulator (SOI) substrates may be advantageous in CMOS technology. In comparison with field-effect transistors built using a bulk silicon wafer, a semiconductor-on-insulator substrate permits operation at significantly higher speeds with improved electrical isolation and reduced electrical losses. Contingent on the thickness of the device layer of the SOI substrate, a field-effect transistor may operate in a partially-depleted mode in which the depletion layer in the channel in the device layer does not extend fully to the buried oxide layer when typical control voltages are applied to the gate electrode.
Partially-depleted SOI field-effect transistors may be fabricated with two types, namely floating-body SOI field-effect transistors (FBFET) or body contacted SOI field-effect transistors (BCFET). A FBFET conserves device area due to its comparatively small size, but suffers from the floating body effect due to the absence of a body contact. A BCFET may be unstable during operation, especially when operating in an RF circuit or a high speed digital circuit, because the threshold voltage is a function of a fluctuating body voltage. A BCFET includes a body contact that may eliminate body effects. However, BCFET covers more chip area than a FBFET, which reduces the density of a circuit built using BCFETs in comparison with a circuit built using FBFETs.
Improved structures for field-effect transistors and methods of forming such structures are needed.